A lithographic apparatus is a Machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                              C          ⁢                                          ⁢          D                =                              k            1                    *                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapour, such as Xe gas or Li vapour. A laser device, such as a CO2 laser, may provide the strong laser beam. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
Debris may be released from the plasma during or after plasma irradiation. Such debris may damage the radiation source. For example, such debris can damage surfaces of optics.
U.S. Pat. No. 6,377,651 B discloses a laser produced extreme ultraviolet (EUV) source based on a water droplet target with an auxiliary electrode system between the source and the collector mirror. The collector mirror collects the EUV radiation produced by the plasma. The auxiliary electrode system creates a repellent electric field, by imposing a DC voltage on the mirror that slows down and reverses the trajectories of ions from the plasma source before they impact the collector mirror.
United States Patent Application Publication No. 2008/0283779 discloses a plasma radiation system with collector mirror that is formed for normal incidence of radiation arranged behind the plasma. An additional mechanical fin arrangement for suppressing debris is arranged between a gas curtain generated and the collector mirror. The gas curtain may comprise a buffer gas for decelerating debris particles, or a gas mixture, which brings about a spectral filtering of the emitted EUV radiation at the same time.